First I'd like to say that Mp3tag is a great program. Very friendly to novice users, and as I learn of these forum also quite powerful for advanced users. And to be honest I'm not that advanced.
I'd like to have a means to write to the local CDDB. Why? I have some home-made compilation CD's (which are obviously not in freedb) but the source CD's are. With all the converters etc. provided by Mp3tag I have managed to have the MP3's correctly tagged - only I have not found a way to get that info into the local CDDB.
Does someone know a way to do this?

Or is it otherwise possible to add this as a feature in a next version?

This has been asked before (in the German part of the support forums) and Florian stated that he does not plan to implement this feature.
You could use Exact Audio Copy to submit your CD information to local freedb.

Unfortunately I understand too little German to read the German forums...
But of course I followed your suggestion and checked EAC.
Both EAC and CDex are capable of writing to local CDDB - but the only way I can find is to type in the data and then write to local CDDB.
What I would like to do is transfer data from ID3tags in MP3 files to local CDDB, to save the typing. All that to make the info available to other applications when I use the audio CD.
Is that really capable with EAC? And is that really the function which Florian does not want to implement?

Have you tried Orange CD from Firetongue. Allows you to import information from MP3's locally to catalogue your collection getting the details from the ID Tags and supports exporting to a local cddb which can then be used with other applications that support local cddb access (eg: Nero 5 & 6). Registration should you like the program is reasonable and a once of fee, not a yearly fee and updates are included. I registered and paid my couple of dollars as the program has been worth it's weight in gold as the old saying goes.